1. Field of the Invention
The present invention relates to an apparatus and method for transmitting a pilot signal in a wireless communication system. More particularly, the present invention relates to an apparatus and method for reducing a pilot overhead in a multi-user multi-antenna system.
2. Description of the Related Art
Various multimedia services for a wireless environment are now being used or developed due to the rapid growth of mobile communication markets. In the wireless environment, a large amount of data is transmitted at a high rate in order to provide various multimedia services. Thus, research is being conducted on a multi-antenna system (e.g., a Multiple Input Multiple Output (MIMO) system) that can use finite frequency resources more efficiently.
In comparison with a single-antenna system, the multi-antenna system can increase the transmission reliability and the transmission rate even without an allocation of additional frequencies or transmit (TX) power by transmitting data through independent channels for the respective antennas. In addition, the multi-antenna system can further increase the frequency efficiency by allowing multiple users to simultaneously share the frequency resources through the spatial resources secured by multiple antennas. Hereinafter, the multi-antenna system allowing multiple users to share frequency resources will be referred to as a multi-user multi-antenna system.
The multi-user multi-antenna system uses a beamforming technique to eliminate inter-user interference. For example, the multi-antenna system may use a coordinated beamforming technique to eliminate an inter-user interference.
If the multi-antenna system uses a coordinated beamforming technique, a transmitter generates a pre-code and a postcode by using the downlink channel information of receivers enabled in a service area. Herein, the pre-code and the postcode have a vector value or a matrix value.
Also, the transmitter transmits an orthogonal dedicated pilot signal to each of the receivers so that the receivers can detect the postcode. For example, in order to transmit the pilot signal to two receivers, the transmitter allocates a pilot tone to each of the receivers by using a subchannel of a tile structure constructed as illustrated in FIG. 1.
FIG. 1 is a diagram illustrating a subchannel structure of a conventional wireless communication system.
Referring to FIG. 1, a wireless communication system constructs a subchannel with tiles arranged in a 4×3 matrix of frequency resources and time resources.
A transmitter allocates the first tone and the twelfth tone for a pilot signal of the first receiver and allocates the third tone and the tenth tone for a pilot signal of the second receiver so that the pilot signals of the receivers are orthogonal to each other.
In addition, the transmitter uses the tones other than the tones allocated for the pilot signals, for data transmission.
As described above, the transmitter allocates the orthogonal pilot signals to the respective receivers. Accordingly, the receiver estimates an effective channel on a stream-by-stream basis by using the pilot signal allocated by the transmitter. The receiver uses the estimated effective channel to generate a matched filter, and uses the generated matched filter as a postcode to detect an interference-free signal. Herein, the matched filter is the postcode that provides the maximum Signal to Interference and Noise Ratio (SINR).
Because the number of the antennas of a receiver is small in the coordinated beamforming technique, the multi-antenna system using the coordinated beamforming technique can simultaneously transmit as many streams as the number of the antennas of a transmitter. Thus, the transmitter recognizes receivers as receive (RX) antennas and simultaneously transmits streams to the receivers, thereby increasing the transmission rate.
However, as the number of the TX streams increases with an increase in the number of the receivers served by the transmitter, the use of resources for dedicated pilot signals allocated to the receivers increases linearly, which increases the ratio of the amount of radio resources used for pilot signal transmission to the amount of radio resources used for data transmission in the multi-antenna system.